"Extra" box appears

When using Lift and Drop to move boxes, if I hit Backspace to undo while the Sokoban is pushing the box, the Sokoban returns to his previous position but an invisible ball is located at the drop point. If I then use Sokoban to move this box, it appears.
I can then complete the level using this box and have an extra box left over.
When I play my solution, the extra box remains invisible, even when pushed, and the hole it occupies does not appear filled. But when the whole solution is played out, an extra box remains.

In fact this is the same bug noted by blackrose.
If I Lift and Drop to the position of the Sokoban and hit Backspace while the push is being executed, then when the Sokoban gets back to his position, he disappears and a box appears. If I use arrows to move the Sokoban, he appears as well as the box.
Pierre